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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_asio.tutorial.tuttimer4"></a><a class="link" href="tuttimer4.html" title="Timer.4 - Using a member function as a completion handler">Timer.4 - Using a member
      function as a completion handler</a>
</h3></div></div></div>
<p>
        In this tutorial we will see how to use a class member function as a completion
        handler. The program should execute identically to the tutorial program from
        tutorial Timer.3.
      </p>
<pre class="programlisting"><span class="special">#</span><span class="identifier">include</span> <span class="special">&lt;</span><span class="identifier">functional</span><span class="special">&gt;</span>
<span class="special">#</span><span class="identifier">include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="special">#</span><span class="identifier">include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">asio</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<p>
        Instead of defining a free function <code class="computeroutput"><span class="identifier">print</span></code>
        as the completion handler, as we did in the earlier tutorial programs, we
        now define a class called <code class="computeroutput"><span class="identifier">printer</span></code>.
      </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">printer</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
</pre>
<p>
        The constructor of this class will take a reference to the io_context object
        and use it when initialising the <code class="computeroutput"><span class="identifier">timer_</span></code>
        member. The counter used to shut down the program is now also a member of
        the class.
      </p>
<pre class="programlisting">  <span class="identifier">printer</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_context</span><span class="special">&amp;</span> <span class="identifier">io</span><span class="special">)</span>
    <span class="special">:</span> <span class="identifier">timer_</span><span class="special">(</span><span class="identifier">io</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">chrono</span><span class="special">::</span><span class="identifier">seconds</span><span class="special">(</span><span class="number">1</span><span class="special">)),</span>
      <span class="identifier">count_</span><span class="special">(</span><span class="number">0</span><span class="special">)</span>
  <span class="special">{</span>
</pre>
<p>
        The <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">bind</span></code> function works just as well with class
        member functions as with free functions. Since all non-static class member
        functions have an implicit <code class="computeroutput"><span class="keyword">this</span></code>
        parameter, we need to bind <code class="computeroutput"><span class="keyword">this</span></code>
        to the function. As in tutorial Timer.3, <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">bind</span></code>
        converts our completion handler (now a member function) into a function object
        that can be invoked as though it has the signature <code class="computeroutput"><span class="keyword">void</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&amp;)</span></code>.
      </p>
<p>
        You will note that the boost::asio::placeholders::error placeholder is not
        specified here, as the <code class="computeroutput"><span class="identifier">print</span></code>
        member function does not accept an error object as a parameter.
      </p>
<pre class="programlisting">    <span class="identifier">timer_</span><span class="special">.</span><span class="identifier">async_wait</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">bind</span><span class="special">(&amp;</span><span class="identifier">printer</span><span class="special">::</span><span class="identifier">print</span><span class="special">,</span> <span class="keyword">this</span><span class="special">));</span>
  <span class="special">}</span>
</pre>
<p>
        In the class destructor we will print out the final value of the counter.
      </p>
<pre class="programlisting">  <span class="special">~</span><span class="identifier">printer</span><span class="special">()</span>
  <span class="special">{</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Final count is "</span> <span class="special">&lt;&lt;</span> <span class="identifier">count_</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
  <span class="special">}</span>
</pre>
<p>
        The <code class="computeroutput"><span class="identifier">print</span></code> member function
        is very similar to the <code class="computeroutput"><span class="identifier">print</span></code>
        function from tutorial Timer.3, except that it now operates on the class
        data members instead of having the timer and counter passed in as parameters.
      </p>
<pre class="programlisting">  <span class="keyword">void</span> <span class="identifier">print</span><span class="special">()</span>
  <span class="special">{</span>
    <span class="keyword">if</span> <span class="special">(</span><span class="identifier">count_</span> <span class="special">&lt;</span> <span class="number">5</span><span class="special">)</span>
    <span class="special">{</span>
      <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">count_</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
      <span class="special">++</span><span class="identifier">count_</span><span class="special">;</span>

      <span class="identifier">timer_</span><span class="special">.</span><span class="identifier">expires_at</span><span class="special">(</span><span class="identifier">timer_</span><span class="special">.</span><span class="identifier">expiry</span><span class="special">()</span> <span class="special">+</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">chrono</span><span class="special">::</span><span class="identifier">seconds</span><span class="special">(</span><span class="number">1</span><span class="special">));</span>
      <span class="identifier">timer_</span><span class="special">.</span><span class="identifier">async_wait</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">bind</span><span class="special">(&amp;</span><span class="identifier">printer</span><span class="special">::</span><span class="identifier">print</span><span class="special">,</span> <span class="keyword">this</span><span class="special">));</span>
    <span class="special">}</span>
  <span class="special">}</span>

<span class="keyword">private</span><span class="special">:</span>
  <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">steady_timer</span> <span class="identifier">timer_</span><span class="special">;</span>
  <span class="keyword">int</span> <span class="identifier">count_</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
        The <code class="computeroutput"><span class="identifier">main</span></code> function is much
        simpler than before, as it now declares a local <code class="computeroutput"><span class="identifier">printer</span></code>
        object before running the io_context as normal.
      </p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
  <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_context</span> <span class="identifier">io</span><span class="special">;</span>
  <span class="identifier">printer</span> <span class="identifier">p</span><span class="special">(</span><span class="identifier">io</span><span class="special">);</span>
  <span class="identifier">io</span><span class="special">.</span><span class="identifier">run</span><span class="special">();</span>

  <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
        See the <a class="link" href="tuttimer4/src.html" title="Source listing for Timer.4">full source listing</a>
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        Return to the <a class="link" href="../tutorial.html" title="Tutorial">tutorial index</a>
      </p>
<p>
        Previous: <a class="link" href="tuttimer3.html" title="Timer.3 - Binding arguments to a completion handler">Timer.3 - Binding
        arguments to a completion handler</a>
      </p>
<p>
        Next: <a class="link" href="tuttimer5.html" title="Timer.5 - Synchronising completion handlers in multithreaded programs">Timer.5 - Synchronising
        completion handlers in multithreaded programs</a>
      </p>
</div>
<div class="copyright-footer">Copyright © 2003-2025 Christopher M.
      Kohlhoff<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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